Field of the Invention
The present invention relates the field of flowmeters for gas/liquid two-phase fluids.
Related Art
Measuring the flow rate of a two-phase fluid composed of a liquid and a gas is a difficult operation when it is desired to measure a mass flow rate. Indeed, all sensors that measure a flow rate are disturbed when they are brought into contact with a two-phase liquid, the density of which changes at any moment. This is in particular valid for measuring the flow rate of cryogenic fluids such as liquid nitrogen.
Certain flowmeters listed in the literature are based on the measurement of the velocity of the fluid. These are for example:                turbine flowmeters: a turbine is installed in the fluid in motion and the rotational speed of the turbine gives a representation of the velocity of the fluid.        Pitot tube flowmeters: two tubes are installed in the fluid in motion to be measured. One tube is installed perpendicular to the flow and gives the static pressure, the other is installed parallel to the flow and gives the total dynamic pressure. The dynamic pressure difference between these two measurements makes it possible to calculate the flow rate.        Ultrasonic flowmeters: some use the Doppler effect (analysis of the frequency reflected by the particles of the fluid that gives a representation of the velocity of the particle and therefore of the fluid) while others measure a difference in transit time of an ultrasonic wave from upstream to downstream and from downstream to upstream (representation of the velocity of the fluid).        
In all these cases, when the density of the fluid varies continuously, the change from volume flow rate to mass flow rate is difficult to carry out accurately.
Other systems use the measurement of head loss (pressure loss) in order to deduce the flow rate therefrom. These are for example calibrated orifice flowmeters that measure the head loss upstream and downstream of a calibrated orifice placed in the fluid in motion. The measurement from these devices is highly disturbed when the fluid does not have a constant density and when the content of gas increases in the liquid.
Electromagnetic flowmeters, applicable only to fluids having a sufficient electrical conductivity, use the principle of electromagnetic induction: an electromagnetic field is applied to the fluid and the electromotive force created (force proportional to the flow rate of the fluid) is measured. In the case of measuring the flow rate of (nonconductive) cryogenic fluids such as liquid nitrogen, this principle is not applicable.
Vortex flowmeters are based on the phenomenon of generation of vortices that are observed behind a fixed bluff body placed in a fluid in motion (Karman effect). Measuring the pressure variations created by these vortices gives the frequency of the vortices, which is itself proportional to the velocity of the fluid when the fluid retains constant properties. When the density of the fluid varies, the measurement is distorted.
Thermal flowmeters are those based on measuring the increase in temperature created by a constant supply of energy. A system with two temperature probes measures the temperature difference between the flow entering and leaving the flowmeter. Between these two probes, a resistance heater provides a known amount of energy. When the heat capacity of the fluid in motion is known, the flow rate may be calculated from these measurements. However, this principle is not applicable to two-phase liquids, of which the thermal behavior (vaporization of the liquid) is completely different from single-phase liquids.
Only a Coriolis mass flowmeter gives an accurate measurement of the mass flow rate of a fluid. The flowmeter consists of a U-shaped or omega-shaped or curved tube in which the fluid circulates. The U-shape is subjected to a lateral oscillation and the measurement of the phase shift of the vibrations between the two arms of the U-shape gives a representation of the mass flow rate. However, its cost is relatively high and when it is used at very low temperatures (liquid nitrogen at −196° C. for example) and with a fluid that has a density that varies enormously and that comprises a large portion in the gas phase, there is a need to highly insulate the system (efficient insulation such as vacuum insulation for example), and despite all that, the measurements are distorted when the gas content exceeds a few percent by mass. It will also be noted that the measurement is often rendered impossible when the velocity of the fluid is low or zero (in the first half of the measurement range).
As may be observed, the measurement of the flow rate of a two-please liquid and in particular the measurement of the flow rate of a cryogenic fluid with an acceptable accuracy is not easy to carry out with the apparatus currently available on the market.
The literature has then proposed other types of solutions, including systems based on the principle of measuring the level of a liquid flowing in a channel just before a restriction of the flow area. This system, described in document US Pat. No. 5 679 905 operates in substance as follows: the two-phase fluid is firstly separated into a gas phase which is not measured and a liquid phase, the flow rate of which is measured. This liquid passes into a channel which has a reduction in cross section at its outlet. The higher the flow rate, the higher the level of liquid in the channel and a measurement of the level in this channel makes it possible to deduce the instantaneous flow rate. As is observed, this system does not take into account the gaseous flow rate which, in certain applications, is negligible. On the other hand, this system makes it possible to measure, with a relatively good accuracy, the liquid flow rate without being disturbed by the gas content, which is the desired objective.
It will be noted in passing that in order for this system to operate correctly, it must be well insulated from heat gains that could vaporize a portion of the insulated liquid and thus disturb the measurement of the level. This is why vacuum insulation is used in this system.
It will also be noted that in order for the system to operate, there must be the presence of two phases in the flowmeter, which prohibits the operation thereof with a subcooled liquid (pure liquid with no gas phase).
In the case where the measurement of the flow rates of liquid and of gas is necessary, use is sometimes made of a system that takes up the same principle of separation of the phases before the flow rate measurement.
Thus, systems have the following device:                the two-phase liquid passes firstly into a phase separator that separates the liquid phase from the gas phase;        the gas phase is sent to a volumetric flowmeter (of turbine type for example) with a temperature compensation;        the liquid phase is also sent to a volumetric flowmeter (of turbine type for example);        these two flow rate measurements are then converted into a mass measurement and are added.        
A priori, this device is more expensive than the preceding one, it may be believed that it will be very accurate. In practice, it is observed that the measurement of the liquid flow rate is marred by errors that fluctuate depending on the pressure and temperature conditions of the liquid entering the flowmeter. These measurement errors are due to the presence of gas in the liquid phase that passes through the flowmeter. Indeed, when the liquid leaves the phase separator in order to go to the flowmeter, a portion of liquid vaporizes, either because of heat gains or because of the drop in pressure due to a rise of the liquid, or because of a drop in pressure due to the head loss created by the flowmeter itself.
Finally, in order to measure the flow rate of a cryogenic liquid, it is also possible to avoid the problems cited above by creating pressure and temperature conditions different from the equilibrium pressure (boiling range). In this field, the method most commonly used is increasing the pressure of the liquid. In practice, a flowmeter will for example be installed at the outlet of a cryogenic pump (high-pressure side). In this case, the liquid is for example pumped into a tank where it is at equilibrium and it is raised in pressure by the pump, with almost no increase in temperature. The pipes and the flowmeter that follow may then create a head loss, this will not result in the liquid vaporizing provided that the head loss is significantly lower than the increase in pressure created by the pump.
In this case, it is possible to use a conventional flowmeter of vortex, turbine or other type insofar as it withstands the low temperatures.
This technique is for example perfectly suitable for the flow rate measurement of nitrogen delivery trucks. It is reliable and has an acceptable cost insofar as the cryogenic pump is required for other reasons.
On the other hand, when it is necessary to measure the flow rate of liquid nitrogen at a point where there is no cryogenic pump, then this technique is no longer advantageous.